British troops in Kandahar, Afghanistan. Their presence has now been extended into next year.
Photograph: Matt Cardy/Getty Images

David Cameron has increased the number of troops serving in Afghanistan for the first time since the UK’s withdrawal began.

At a Nato summit in Poland, the prime minister’s deputy official spokesman said the UK would be increasing the size of its mission from 450 to 500 and delaying the departure of the last remaining forces in the country into 2017.

Their presence has now been extended into next year and the size of the mission enhanced, in a move suggesting the Afghan army is still not ready to withstand the Taliban without support from international military allies such as the US and the UK. “The aim is to build the capacity of the Afghan national defence and security forces,” Cameron’s spokesman said.

Cameron fought the 2010 election promising total withdrawal of UK troops from Afghanistan after a decade-long war started in the aftermath of the 9/11 terror attacks in the US. He declared in 2013 that it would be mission accomplished for British forces after the end of the troop drawdown the following year.

His spokesman on Friday denied the UK was getting drawn back into the conflict, which has cost more than 450 British lives. “It is a Nato mission,” he said. “It’s not mission creep. They are not engaged in a combat capacity. They are engaged in training and mentoring.”

About 21 of the additional forces will reinforce the counter-terrorism mission, 15 will strengthen the Afghan army in a leadership development role and 13 will be sent to the Nato support headquarters. The UK government confirmed its financial support amounting to £70m a year would continue until 2020, with another £178m for the Afghan government in 2017.

Britain is not the only country that has extended the stay of its troops in Afghanistan. Last year, Barack Obama delayed the departure of US troops from the country, saying a force of 5,500 would stay beyond the end of his term of office in 2017.

At the peak of the conflict, the UK had about 9,500 service personnel in the country.